This invention relates to bellows heat exchangers for cardiovascular surgery equipment, and more particularly to efficiency improvements in such bellows heat exchangers.
In cardiovascular surgery, a heart-lung machine provides an extracorporeal circuit in which the patient""s blood is pumped and oxygenated while the heart is stopped. Because temperature control of the blood returned to the patient is important, a heat exchanger is typically provided in the circuit either as a separate unit or as part of the oxygenator. In a typical heat exchanger, water at a controlled temperature is conveyed along one side of a heat-conducting interface while blood is conveyed along the other side.
A problem of the prior art is that the conventional method of forming the bellows produces pleats in which fluid flowing into the pleat from either side must pass through a neck that is narrower than the fluid chamber formed by the pleat. This limits the efficiency of fluid penetration into the pleat, and therefore the heat transfer capacity of the bellows. In order to improve the exchange capacity of the bellows, it would be desirable to provide a manufacturing process which would eliminate this narrow neck.
Also, a problem occurs in conventional pleated heat exchangers due to the failure of the inner core of the heat exchanger to distribute water evenly over the full length of the bellows. One type of conventional core consists of a cylindrical body in which a water manifold having the same diameter as the inlet fitting is formed. This manifold is connected to the outside of the core through slots that extend longitudinally of the core. It has been found that this construction results in a kinetic energy distribution that causes an uneven flow out of the slots, particularly near the distal, closed end of the manifold.
The present invention enhances the heat exchange capacity of a pleated metal bellows type heat exchanger by providing an even, unobstructed fluid flow into the pleats of the bellows so as to maximize the amount of fluid which comes into contact with the heat exchange surface of the bellows. Specifically, the invention enhances fluid penetration into the pleats by so forming the pleats that the fluid path is widest at the mouth of the pleat and continuously narrows toward the inner end of the pleat""s fluid chamber. In another aspect, the invention provides a water manifold structure that dissipates the kinetic energy of the water introduced into the heat exchanger and produces an even water flow into the pleats throughout the length of the bellows.
More particularly, there is provided a heat exchanger for controlling the temperature of blood in an extracorporeal circuit, which comprises a metal heat exchanging bellows having a first surface adapted for contact with blood and a second, separate surface adapted for contact with water. The metal bellows comprises a plurality of pleats defining at first set of fluid chambers on the first surface and a second set of fluid chambers on the second surface, wherein each fluid chamber in each of the first and second sets of fluid chambers has a mouth and an interior end, and is so shaped such that it is widest at the mouth and narrowest at the inner end. The inventive heat exchanger further comprises a water inlet and a manifold extending from the water inlet for directing water into a portion of the second set of fluid chambers through at least one slot.
In a further aspect of the invention, the fluid chambers in the first set of fluid chambers are substantially wider than the fluid chambers in the second set of fluid chambers. Preferably, the heat exchanger further comprises a housing, a blood inlet, and an annular space adapted to receive blood from the inlet, wherein the annular space is disposed between a radially outward side of the bellows and a wall of the housing. Portions of the first set of chambers are adapted to receive blood from the annular space.
In a preferred embodiment of the invention, the aforementioned pleats, in cross-section, have substantially the shape of a rectified sine wave, and each of the fluid chambers in each of the first and second sets of fluid chambers narrows continuously from its mouth toward its inner end. The heat exchanger further comprises a substantially cylindrical core, wherein the metal heat exchanging bellows surrounds the core and is substantially coaxial therewith. The water inlet comprises an inlet connector, and the manifold is disposed in the core. A cross-sectional area of the manifold is substantially larger than a cross-sectional area of the inlet connector, and includes a major lobe and a minor lobe interposed between the major lobe and the slot. The major lobe has a cross-sectional area substantially larger than the cross-sectional area of the inlet connector.